Mechanical joints have in a short time taken great market shares mainly owing to their superior laying properties, joint strength and joint quality. Even if the floor according to WO 9426999 as described in more detail below and the floor marketed under the trademark Alloc© have great advantages compared with traditional, glued floors, further improvements are, however, desirable.
Mechanical joint systems are very convenient for joining not only of laminate floors but also wooden floors and composite floors. Such floorboards may consist of a large number of different materials in the surface, core and rear side. As will be described below, these materials can also be included in the different parts of the joint system, such as strip, locking element and tongue. A solution involving an integrated strip which is formed according to, for example, WO 9426999 or WO 9747834 and which provides the horizontal joint, and also involving a tongue which provides the vertical joint, results, however, in costs in the form of material waste in connection with the forming of the mechanical joint by machining of the board material.
For optimal function, for instance a 15-mm-thick parquet floor should have a strip which is of a width which is approximately the same as the thickness of the floor, i.e. about 15 mm. With a tongue of about 3 mm, the amount of waste will be 18 mm. The floorboard has a normal width of about 200 mm. Therefore the amount of material waste will be about 9%. In general, the cost of material waste will be great if the floorboards consist of expensive materials, if they are thick or if their format is small, so that the number of running meters of joint per square meter of floor will be great.
Certainly the amount of material waste can be reduced if a strip is used which is in the form of a separately manufactured aluminum strip which is already fixed to the floorboard at the factory. Moreover, the aluminum strip can in a number of applications result in a better and also more inexpensive joint system than a strip machined and formed from the core. However, the aluminum strip is disadvantageous since the investment cost can be considerable and extensive reconstruction of the factory may be necessary to convert an existing traditional production line so that floorboards with such a mechanical joint system can be produced. An advantage of the prior-art aluminum strip is, however, that the starting format of the floorboards need not be changed.
When a strip produced by machining of the floorboard material is involved, the reverse is the case. Thus, the format of the floorboards must be adjusted so that there is enough material for forming the strip and the tongue. For laminate floors, it is often necessary to change also the width of the decorative paper used. All these adjustments and changes also require costly modifications of production equipment and great product adaptations.
In addition to the above problems relating to undesirable material waste and costs of production and product adaptation, the strip has disadvantages in the form of its being sensitive to damage during transport and installation.
To sum up, there is a great need of providing a mechanical joint at a lower production cost while at the same time the aim is to maintain the present excellent properties as regards laying, taking-up, joint quality and strength. With prior-art solutions, it is not possible to obtain a low cost without also having to lower the standards of strength and/or laying function. An object of the invention therefore is to indicate solutions which aim at reducing the cost while at the same time strength and function are retained.
The invention starts from known floorboards which have a core, a front side, a rear side and opposite joint edge portions, of which one is formed as a tongue groove defined by upper and lower lips and having a bottom end, and the other is formed as a tongue with an upwardly directed portion at its free outer end. The tongue groove has the shape of an undercut groove with an opening, an inner portion and an inner locking surface. At least parts of the lower lip are formed integrally with the core of the floorboard and the tongue has a locking surface which is designed to coact with the inner locking surface in the tongue groove of an adjoining floorboard, when two such floorboards are mechanically joined, so that their front sides are located in the same surface plane (HP) and meet at a joint plane (VP) directed perpendicular thereto. This technique is disclosed in, inter alia DE-A-3041781, which will be discussed in more detail below.
Before that, however, the general-technique regarding floorboards and locking systems for mechanical locking-together of floorboards will be described as a background of the present invention.